A machine tool that performs processing by rotating a rotary shaft may generate a so-called “chatter vibration” during processing when processing conditions such as the depth of cut and the rotation speed of the rotary shaft are inappropriate. Chatter vibration may reduce the finishing accuracy of a processed surface, and may break the tool. Thus, it is desired to suppress chatter vibration.
There are two types of chatter vibration, namely, a “regenerative chatter vibration” which is self-excited vibration generated between the tool and the workpiece, and a “forced chatter vibration” generated by the machine tool itself. Prior to the present application, the applicant proposed a vibration suppressing device that differentiates between the two types of chatter vibration so that respective countermeasures can be taken for the two types of chatter vibration (Japanese Patent Application Publication No. JP 2008-290186 A). In the vibration suppressing device described in JP 2008-290186 A, a time-domain vibration acceleration detected by vibration sensors is subjected to an FFT analysis to calculate a frequency-domain vibration acceleration, and to calculate a chatter frequency FC at which the frequency-domain vibration acceleration becomes maximum. Then, a k′ number, a k number, and a phase difference ε are calculated using formulas (1) to (3) below to determine forced chatter vibration when the phase difference ε is close to zero (e.g., 0.1 or less), that is, the k′ number is close to an integer, and to determine regenerative chatter vibration otherwise.k′=60×FC/(Z×S)  (1)k=|k′|  (2)ε=k′−k  (3)
In formula (1), Z is the number of tool flutes, and S is the rpm of the rotary shaft. In formula (2), |x′| is a floor function that represents the largest integer that is less than x (that is, formula (2) derives the integer part of the k′ number).
The vibration suppressing device described in JP 2008-290186 A determines whether the phase difference ε is close to zero based on whether the phase difference ε falls within a range whose upper and lower limits are defined by two constants. That is, as seen in FIG. 6, which shows the relationship between the rotation speed of the rotary shaft and the chatter frequency of the chatter vibration, a technique using the k′ number according to the related art determines forced chatter vibration in ranges defined by two dotted lines that are separated by a solid line. Thus, as is clear from FIG. 6, the ranges where forced chatter vibration is determined are proportional to the rotation speed and the chatter frequency, and are wide in regions where both the rotation speed and the chatter frequency are high. Therefore, under such processing conditions, regenerative chatter vibration with a phase difference ε of close to zero may be erroneously determined as forced chatter vibration.
The detection accuracy of vibration acceleration and rotation speed is limited, and inevitably subjected to error. If the rotation speed detection resolution is defined as ΔS and the frequency resolution in an FFT calculation device is defined as Δf, a maximum error εerr that may occur in the phase difference ε is calculated using formula (4) below. That is, processing conditions in which at least one of the rotation speed and the chatter frequency is low, for example, have narrow ranges for determination of forced chatter vibration. Under such processing conditions, the phase difference ε may exceed the ranges whose upper and lower limits are defined by two constants because of the error εerr. That is, forced chatter vibration may be erroneously determined as regenerative chatter vibration.
Forced chatter vibration is further divided into two types, namely, a “flute-passage-period forced chatter vibration” generated at an integer times the period during which flute portions of the tool pass over a surface of the workpiece, and a “rotational-period forced chatter vibration” generated in the rotational period of the rotary shaft. Flute-passage-period forced chatter vibration can be determined by the vibration suppressing device described in JP 2008-290186 A above. For rotational-period forced chatter vibration, the fundamental frequency is the rotational period, and a phase difference εsp is calculated using formula (5) below. Thus, chatter vibration with an order of vibration N that is not a multiple of the number of flutes Z does not have a phase difference ε(εsp) of zero, and therefore cannot be detected as forced chatter vibration.
[Expression 1]
Error in phase difference εεerr=((FC+Δf)×60)/(Z×(S−ΔS))+((FC−Δf)×60)/(Z×(S+ΔS))  (4)
Phase difference in rotational-period forced chatter vibrationεsp=Z/N  (5)
In formula (4), FC is the chatter frequency, and S is the rpm of the rotary shaft.